U.S. Pat. No. 6,263,868 granted Jul. 24, 2001, to J. Koch et al, describes a gas burner with two flame rings positioned at different heights, namely a main flame ring positioned at a higher level and a simmering flame ring positioned at a lower level. However the simmering flame ring is disposed on the circumferential wall of the burner and is sensitive to those air movements which inevitably occur on the cooking hob. Moreover, the primary air feeding the simmering flame ring originates from below the cooking hob with the result that this ring, already fed with a minimum gas flow and sensitive to air movements on the cooking hob, is also sensitive to the streaming effect, i.e. the pressure reduction to which the primary air flow is exposed following opening of a door positioned below the cooking hob, which can result in extinguishing of the simmering flames.
Another gas burner with two flame rings, positioned at different heights, namely a main flame ring for high temperature cooking and a simmering ring flame for heating, is shown in applicant's co-pending application Ser. No. 11/795,751, presently pending Group Art Unit 3749.